Resistance Seam Welding:
In principle seam welding is similar to spot welding except that it uses disc shaped electrodes. A current impulse is applied through the rollers to the material in contact with them. The heat generated thus rollers to the material in the pressure from the electrodes completes the weld. If the current is put OFF and ON quickly, a continuous fusion zone made up of overlapping nuggets is obtained and the process is known as Stitch Welding Process.
Unlike spot welding the disc shaped electrodes are not separated after each weld, but maintain continues pressure over the work pieces. The electrode current is timed to flow in pulses so that a row of welds is produced along the interface. Copper alloy electrodes are used to keep the heat at the electrode contact surface to a minimum. The motion of the electrodes and current impulses are so arranged that the weld nuggets overlap forming a gas or liquid pressure tight weld.
The amount of overlap depends on the ratio of current ON and OFF time. The normal overlap is generally 25 to 50 percent.
After the welding is over the electrodes and work pieces are flooded with water to dissipate the heat.
Types of seam welding:
Seam welding may be divided in two main categories namely:
1. Continuous motion seam welding
2. Intermittent motion seam welding
Continuous motion seam welding:
In the continuous motion method of seam welding the electrodes are rotated at a predetermined constant speed and the current impulses timed to get an overlapping weld. Alternatively the work pieces may be moved at constant speed with the electrodes idling under the welding pressure.
Intermittent motion seam welding:
In the intermittent method the work pieces move till the weld position stop for the welding to take place automatically move to the next weld position, stop again for welding and so on. This method is suitable for seam welding in thicker sheets which are too thick to be properly welded by the continuous motion method.
Resistance Seam Welding Wheel:
Types of seam welding joints:
The common types of seam welding joints are:
1. Lap seam weld (Common type weld)
2. Butt seam weld
Advantages of seam welding:
1. A continuous overlapping weld produced by the process makes it suitable for joining liquid or gas tight containers and vessels
2. Efficient energy use
3. Filler metals are not required. Hence no associated fumes or gases. This results in clean welds.
4. Roll welding simply joins two work pieces whereas stitch welding produces gas tight and liquid tight joints.
Disadvantages of seam welding:
1. Requires complex control system to regulate the ravel speed of electrodes as well as the sequence of current to provide satisfactory overlapping welds. The welding speed, spots per inch and timing schedule are all dependent on each other
2. Difficult to weld metals having thickness greater than 3mm
3. Relatively higher current is thus required for seam welding than for spot welding.
4. The work pieces to be welded are over lapped sufficiently to prevent metal flowing out from the edges of the pieces during welding under pressure.
Applications of seam welding:
1. Used to fabricate liquid or gas tight sheet metal vessels such as gasoline tanks, automobile mufflers and heat exchangers.
2. The production of seam welded pipes and tubing (Butt seam weld).